In many industrial processes, undesirable excess heat is removed by the use of heat exchangers in which water is used as the heat exchange fluid. The term "cooling water" is applied wherever water is circulated through equipment to absorb and carry away heat. This definition includes air conditioning systems, engine jacket systems, refrigeration systems as well as the multitudes of industrial heat exchange operations, such as those found in oil refineries, chemical plants, steel mills, etc.
Preventing the corrosion of industrial heat transfer equipment is essential to the efficient and economical operation of a cooling system. Excessive corrosion of metallic surfaces can cause the premature failure of process equipment, necessitating downtime for the replacement or repair of the equipment.
Additionally, the buildup of corrosion products on heat transfer surfaces impedes water flow and reduces heat transfer efficiency, thereby limiting production or requiring downtime for cleaning, and can also cause rapid localized corrosion and sub sequent penetration of metallic surfaces through the formation of differential oxygen concentration cells. The localized corrosion resulting from differential oxygen cells originating from deposits is commonly referred to as "under-deposit corrosion". "Galvanic corrosion" can also occur if the corrosion products include metals different from that of the metal surface.
Chromate compounds, for years, provided protection to cooling water metallurgies, particularly when used in conjunction with polyphosphates, zinc and orthophosphates. With the advent of federal, state and municipal environmental controls, however, chromate became suspect for its environmental impact on lakes, streams, ponds and the like, where it might be discharged. Some industries, such as the petrochemical, petroleum, steel and chemical industries, chose to continue the use of chromates. This use necessitated high capital expenditures for either chromate removal or recovery systems, or for disposing of reduced chromate obtained by the natural treatment of effluents.
Further compounding the corrosion problem in the absence of chromates and the subsequent oxide film formed on the metal surfaces is the introduction of corrosive agents during the course of the cooling operation. These corrosive agents may include, for example, hypochlorite ions added for their biocidal action, sulfide ions present through process leaks, sulfate or chloride ions added as their hydrogen acids to control pH, or corrosion products that are dissolved, dispersed, or redeposited throughout the system. Additionally, higher temperature aqueous systems experience faster corrosion rates due to higher dissolved oxygen levels and a tendency for corrosion inhibiting chemicals to deteriorate.